The company has been involved in the development of the port of Felixstowe since its early beginnings in the 1950's through to the present day when it is Britain's largest and most efficient container port. At the start of this process, the port consisted of a small silted up basin surrounded by undeveloped salt marsh. The port is divided into 5 separate Terminals, each of which can be considered to be a Green Field development in its own right.

The company has been responsible for the design of all the maritime developments at the port, including over 2 km of deep water container quay. Terminals developed at Felixstowe include the Languard Terminal, Dooley Terminal, Trinity 1, Trinity 2 and Trinity 3. To keep pace with its continued development the firm has been responsible for the extension and improvement of the port's links with both the highway and rail networks.
The Felixstowe multi-user oil jetty was designed in the early 60's. The products include petroleum, gas oil, ethyl alcohol and liquid propane. Posford Haskoning designed fire fighting installation for the facility.
The various dredging and reclamation works which have generally formed part of the overall development projects for the expansion of the Port of Felixstowe, have involved over 15 million cubic metres of dredging and approximately 120 ha of reclamation. Considerable cost savings were achieved in the reclamation works by filling over the existing marshy foreshore and silt seabed with due allowance being made for the anticipated settlement in the designs of warehouses, surfacing, drainage and other facilities.